Filters, such as bandpass filters, have numerous applications in communications and electronics. In wireless multiple access communications, a given frequency band must accommodate many wireless users. To accommodate so many users, stringent bandpass filtering requirements are required to minimize interference from communications occurring in neighboring frequency bands or channels.
Conventionally, wireless handsets use fixed-tuned bandpass filters (BPFs) to meet their filtering specifications. The design of such filters is complicated because they must achieve the lowest possible passband insertion loss (IL), while simultaneously achieving a specified large out-of-band rejection. As a specific example, consider full band PCS CDMA handsets using fixed bandwidth filters. The PCS transmit (Tx) band should have no more than −3.5 dB IL inband (1850 to 1910 MHz in the US), while having at least a 38.0 dB out-of-band rejection in the receive (Rx) band (1930 to 1990 MHz range).
Further, this BPF is constrained by size limitations, as manufacturers are continually attempting to manufacture smaller wireless devices. A typical height constraint for a conventional handsets may be 4.0 mm, or less. To meet these demanding electrical requirements, yet possess the smallest possible size and height, high order fixed-tuned filters constructed from either individual coaxial resonator elements or monoblock structures are usually necessary. In addition, to satisfy out-of-band rejection requirements, a transmission zero is usually required, increasing IL at the band edge. Because of variations in ceramics and fabrication tolerances, vendors must individually adjust the characteristics of fixed-tuned filters during their manufacture, driving costs higher.
Moreover, if more than one frequency band is to be supported (e.g., supporting the PCS bands in the U.S., Korea, and India) multiple fixed-tuned BPFs are necessary, requiring extra switches that introduce additional losses. This is true, even if the power amplifier and low noise amplifier used have sufficient bandwidth to operate over these multiple bands.
A tunable BPF permits the BPF to be used over several bands. That is, a lower order filter with a narrow bandpass can be used to selectively tune within a larger passband. To provide tunability in a tunable BPF, a component capable of providing a variable capacitance is typically used.
Several structures are presently used to implement a variable capacitor. For example, movable parallel plates have been used for many years as the tuner in home radios. However, such plates are far too bulky, noisy, and impractical for use in most modern applications. Another alternative, the electronic varactor, is a semiconductor device that adjusts capacitance responsive to an applied voltage. Because the varactor is typically noisy and lossy, particularly in applications above 500 MHz, it is ineffective for high-frequency, low-loss applications where high performance is required.
Another alternative, a microelectromechanical system (MEMS) device can be used to switch between capacitors, responsive to an applied control signal. These devices have not yet proven practical for high-volume low-cost manufacturing. Further, such a mechanism still only provides discrete tuning, between a finite number of fixed capacitor values.
Ferroelectric tunable capacitors are another alternative. Ferroelectric (FE) materials are a class of materials, typically ceramic rare-earth oxides, whose prominent feature is that their dielectric constant (K), and as a consequence, the electric permittivity (ε) changes in response to an applied slowly varying (DC or low frequency) electric field. The relationship of the dielectric constant and the electric permittivity of a material is given as follows:ε=KεO 
where εO is the electric permittivity of a vacuum. At present, there are several hundred known materials that possess FE properties. In a typical FE material, one can obtain a change in dielectric constant as great as approximately 3:1. The DC voltage required to generate such changes depends, in one aspect, upon the dimensions of the FE material over which a DC control voltage is applied. As a result of their variable dielectric constant, one can make tunable capacitors using FE materials, because the capacitance of a capacitor depends on the dielectric constant of the dielectric proximate the capacitor conductors. Typically, a tunable FE capacitor is realized as a parallel plate (overlay), interdigital (IDC), or a gap capacitor.
Conventional FE variable capacitors use a layer of an appropriate FE material, such as barium strontium titanate, BaxSr1−xTiO3 (BSTO), disposed adjacent to one or both conductors of a capacitor. Depending upon the strength of the electric field applied to the FE material and the intrinsic properties of the FE material selected, the capacitance changes. Typically, below the Curie temperature (TC) of the FE film, the FE material is in the ferroelectric state and will exhibit hysteresis in its response to a changing electric field. Above TC, the FE material is in the paraelectric state and will not exhibit hysteresis. Thus, an FE material is generally chosen that has a TC lower than the expected operating temperature so as to operate in the paraelectric state, avoiding the hysteresis effects of the ferroelectric state.
However, conventional FE variable capacitors have proven to be too lossy for use in insertion-loss-sensitive applications such as in the RF circuits of handsets. Moreover, these devices often perform unpredictably, preventing optimal design, construction, and use of FE tunable filters.
Duplexers are used in wireless telephone technology to separate the Tx and the Rx frequencies into their respective signal paths. Duplexers typically comprise two bandpass filters. Each filter “selects” either the Tx or the Rx frequency signal to be passed. The filters are coupled together at one end, forming a common port. This common port is typically coupled to an antenna or a diplexer for sending transmit signals and receiving receive signals.
Strict insertion loss and out-of-band rejection requirements are the primary requirements that influence the design of duplexers for use in loss-sensitive applications, for example, in wireless handsets. Other electrical and mechanical specifications must also be satisfied, such as, for example, size and height requirements.
It would be advantageous if a bandpass filter could be made tunable, to operate at a number of channels within a frequency band.
It would be advantageous if the tunable bandpass filter could be tuned to operate in multiple frequency bands.
It would be advantageous if the above-mentioned bandpass filter could be fabricated using an FE tunable capacitor.
It would be advantageous if a duplexer could be made using two tunable bandpass filters.